Boiling water reactors (BWRs) need to be inspected for purposes of safety and maintenance. Particularly, the welds of the core shroud of the BWR need to be inspected in order to identify intergranular stress corrosion cracking (IGSCC). Inspections are commonly performed during outages of a reactor unit using visual inspection methods and ultrasonic probes. The ultrasonic probes measure the length and depth of cracks in the welds.
Most BWRs include various structures such as jet pumps that make it difficult for currently available inspection devices to access certain weld locations on the core shroud. Also all BWR's are unique making it difficult for a single tool to be able to inspect the entire BWR fleet. BWR core shrouds contain different diameter barrels which decrease as depth increases. The resulting ledges and variation in diameters make accessibility a challenge.
Inspection tooling is generally rigorously designed to minimize the chance for debris to be dropped or lost in the reactor. Most inspection tooling is custom designed to maximize accessibility while preventing loss of material and minimizing breakdowns. Breakdowns of tooling can be costly in terms of increased time to inspect the BWR.
Remotely operated vehicles (ROV) are generally too large to be used to inspect the core shroud, mostly due to the fact that they must be submersible to one hundred feet. Thrust fans are commonly used on an ROV to hold it against the core shroud. The thrust fans that are needed to produce the required thrust force increase the size of the ROV thereby limiting its accessibility. Thrust fans also increase the number of moving parts to risk both failure and potential for loss of parts in the reactor.